Error control enables a data receiver to perform error detection and possibly also error correction in case of transmission errors. Techniques for error control are thus of general importance in any environment in which data are transmitted. Some environments are more prone to errors than others; for example, wireline communications will generally produce less errors per given amount of data than wireless communications. Error control schemes are available which have proven particularly useful in the one or the other environment.
For example, a class of data transmission schemes for error-prone transmission environments is represented by the Automatic Repeat Request (ARQ) “retransmission” schemes, which are utilized particularly in the field of mobile networks. In ARQ, after each transmission of a data block the transmitter waits for a feedback from the receiver before sending the next data block. The receiver will automatically request a repeated transmission of the data block in case of data corruption. In mobile networks conforming to the UMTS Release 5 or Release 6 standard, Hybrid ARQ (HARQ) transmission schemes are employed, in which the data transmission in a data channel is complemented by a transmission of (error) control information in a parallel control channel. This has been standardized in the UMTS Release 5 for HSDPA (High Speed Downlink Packet Access) transmission schemes and in UMTS Release 6 for E-DCH (Enhanced Uplink Dedicated Channel) transmission schemes.
The uplink control channel for E-DCH HARQ is called E-DPCCH (Enhanced uplink Dedicated Physical Control Channel) and carries control information such as an indication of the transport format selected for the packet data transmitted on the parallel uplink data channel (called E-DPDCH, Enhanced uplink Dedicated Physical Data Channel), an indication of the number of HARQ re-transmissions and a so-called happy bit which indicates if the transmitter, e.g. a mobile terminal called User Equipment (UE) in the UMTS environment, could use additional resources if scheduled. The control information increases the probability for successful reception of the data packets transmitted on the E-DPDCH.
Besides error handling, the usage of transmission resources is a general problem in wireless environments and in particular for uplink transmissions, i.e. transmissions in the direction from a terminal to a base station of the network. Terminals such as mobile phones, notebooks, handhelds, etc. generally have limited power resources, for example because of their limited power saving capabilities or because their maximum transmission power is limited due to regulations. Because of these restrictions, the available transmission power for a terminal is limited to a few Watts only in GSM or UMTS networks, whereas a base station may transmit with a power of tens of Watts. Therefore, successful reception of a data transmission at the base station cannot in general simply be afforded by increasing the transmission power. Instead, the terminal is required to efficiently use its available transmission resources while at the same time ensuring a reliable transmission, i.e. a successful reception of the transmitted data. A problem with existing HARQ schemes is that they require in some situations particularly many transmission resources for ensuring a reliable transmission, as will be discussed now.
For a HARQ transmission, each of the transmitted channels requires a particular amount of transmission power to ensure successful reception of each channel at the receiver. This channel-specific transmission power is determined according to predefined rules in the base station and/or the terminal and thus represents a “desired” transmission power. For example, in E-DCH, the desired transmission powers for the E-DPCCH and E-DPDCH, respectively, are determined relative to an uplink pilot channel (the so-called DPCCH, to be distinguished from the E-DPCCH), which is power-controlled by the Node B (the base station of the UMTS network). Whereas the power offset for the E-DPCCH relative to this pilot channel is constant, the actual power offset for the E-DPDCH depends on the chosen transport format (and some further parameters). In other words, the relative cost of the E-DPCCH increases as the offset used for the E-DPDCH decreases.
In more detail, the relative cost of the control channel is highest in cases where only few data have to be transmitted on the data channel, because the control channel carries a fixed amount of control information. As an example, in a E-DCH scheme the transport format with the largest and smallest data block sizes may be of order 10 kbits and some 10 bits, respectively (see, e.g., 3GPP TS 25.321, Annex B). The control information carried on the E-DPCCH may comprise 10 bits in each case. Thus the control information requires comparable resources as the data itself if one of the smallest transport formats is utilized. Small sized transport formats for the uplink data channel may generally be used in case of non-optimal transmissions conditions, e.g. if the terminal is located at a cell border.
In case the available transmission power in the transmitter is smaller than the desired transmission power for the data channel plus the desired transmission power for the control channel (plus possibly a desired transmission power for further channels), the transmitter is in a power-limited regime. In this regime it is particularly important to efficiently use the available transmission power and thereby ensure a reliable data transmission. Further predefined rules are applied in the power-limited regime to allocate the available transmission power to the channels. In E-DCH, the gain factor for the E-DPDCH may for example be reduced such that the relative overhead of the control channel is further increased.
The transmitter runs into a power-limited regime in particular for difficult transmission conditions when the desired transmission powers are high. And it is just in these cases that the relative fraction of the available transmission power allocated to the control channel is largest or, in other words, the relative fraction of the available transmission power allocated to the uplink data channel is lowest, which correspondingly decreases the probability of successful reception of the data. This in turn increases the probability that additional HARQ re-transmissions are required. Therefore the reduced transmission power for the uplink data channel is compensated using additional transmission time.
There exists therefore a need for a technique for uplink data transmission schemes which efficiently use available transmission resources for a reliable transmission.